Debris removal method and assembly

ABSTRACT

A method of removing debris from a well is disclosed, comprising RIH to required depth with coiled tubing having at least one closed check valve, moving a sleeve through the check valve to hold it open, opening a flow control valve located on the coiled tubing below the check valve; then reverse circulating a fluid to entrain debris in the wellbore and bring it to surface through the coiled tubing, before releasing the sleeve from the check valve; and POOH with the at least one check valve returned to the closed configuration. By holding the check valves open, reverse circulation can be performed in the well using coiled tubing, so sufficient lift can be achieved to bring the debris to surface even in underbalanced or low pressure wells. Safety measures can be maintained by having at least one check valve in the conventional closed configuration on RIH and POOH. The flow control valve allows the check valve to be opened or closed without pumping any fluid into the well and thus the sleeve can be moved to hold the check valve open with a relatively low pressure.

The present invention relates to a debris removal method and assembly for use on coiled tubing in an under-balanced well. More particularly, the invention relates to a method and apparatus for clearing debris from a wellbore by reverse circulating. Embodiments of the invention are particularly useful in underbalanced wells, or wells which have a low formation pressure, although embodiments of the invention can permit more efficient removal of debris in many other well conditions. Embodiments of the invention are particularly useful for wellbore cleaning operations, for example after drilling operations.

During production of a well, debris can build up in the wellbore, making efficient production of the well increasingly difficult. This debris may be sand from the formation, scale, metal shavings and perforation debris, all of which can collect in the wellbore and interferes with production. Debris is typically cleared from wellbores during the production phase and during the drilling phase.

One method of removing debris from a wellbore involves the introduction of a fluid which is circulated in the well. For example, cleaning fluid can be pumped down the wellbore through a pipe string and convey debris to the surface of the well upon return through an annulus formed between the pipe string and the wall of the wellbore. Nitrogen or some other gas can be added to the fluid to create a foam for increasing the debris-carrying ability of the fluid. The introduction of nitrogen is also useful in cleaning processes carried out on wells which have a low formation pressure and cannot support a column of liquid.

An alternative to pumping fluid down the pipe string and recovering fluid via the annulus is to use reverse circulation. In reverse circulation, the fluid is pumped down the annulus and returns up the bore of the work string.

Reverse circulation is problematic in work strings deployed on coiled tubing. Coiled tubing has certain advantages over jointed pipe, as it can be run-in-hole (RIH) and pulled out of the hole (POOH) in faster time scales to provide minimal intervention. Coiled tubing is a single length of continuous, un-jointed tubing spooled onto a reel for storage in sufficient quantities to exceed the length of the borehole. Although the coiled tubing may be metal coiled tubing, typically the coiled tubing is composite coiled tubing.

Safety measures dictate that coiled tubing can only be RIH or POOH with the incorporation of check valves located in the bore. These check valves typically comprise one or more flapper valves mounted in the coiled tubing as the prime safety barrier to prevent back flow up the coiled tubing should control of the pump or ancillary surface equipment be lost at surface for any reason. These valves naturally prevent reverse circulation.

According to a first aspect of the present invention there is provided a method of removing debris from a well, the method comprising: RIH to required depth with coiled tubing having at least one check valve arranged in a closed configuration; at depth, applying a first pressure to move a sleeve through the at least one check valve to hold the valve(s) in an open configuration; applying a second pressure, typically greater than the first pressure, to open a flow control valve located on the coiled tubing below the check valve(s); reverse circulating a fluid to entrain debris in the wellbore and bring it to surface through the coiled tubing; releasing the sleeve from the at least one check valve; and POOH with the at least one check valve returned to the closed configuration.

Optionally the sleeve can be moved by a first pressure that can be higher than the formation pressure. The first pressure can typically be a relatively low pressure. The sleeve is typically moved through the check valve by the first pressure acting on a piston area of the sleeve. Typically the sleeve has two piston areas having different surface areas, whereby pressure applied to the sleeve causes a differential piston effect to move the sleeve.

Typically the sleeve can be moved back to the original configuration free of the valve by a third pressure, which can optionally be similar to the first pressure.

Typically the movement of the sleeve is controlled by a pin moving in a slot. Optionally the slot can be a J-slot arrangement, typically with more than one stop, representing different axial positions of the sleeve.

By holding the check valves in an open configuration, reverse circulation can be performed in the well using coiled tubing. Thus sufficient lift can be achieved to bring the debris to surface. Safety measures can be maintained by having at least one check valve in the conventional closed configuration on RIH and POOH.

The flow control valve typically allows the at least one check valve to be opened or closed without pumping any fluid into the well and thus the sleeve can be moved to hold the check valve(s) open with a relatively low pressure.

Optionally a plurality of check valves are provided. This ensures at least two barriers and enhances safety during RIH and POOH. In one embodiment the check valves comprise a dual flapper arrangement.

Optionally, the first pressure is bled off to lock the check valves in the open configuration.

Optionally the flow control valve is a two-way valve opening at the predetermined second pressure to allow flow of fluid from the coiled tubing to the wellbore or to allow free flow from the wellbore to the coiled tubing when the check valves are locked open. Optionally the flow control valve incorporates a separate reverse check valve also.

Optionally, the method further includes the step of closing the flow control valve at depth to retain the debris within the coiled tubing prior to POOH. In this way, a bailer can be created to carry debris to the surface.

Optionally a predetermined volume of debris-laden fluid is pumped into the coiled tubing. In this way, the debris is held between the check valves and the flow control valve, so that it is contained for easier disposal when the coiled tubing is POOH.

Optionally, the method further includes the step of introducing a cleaning fluid into the coiled tubing between the flow control valve and the check valves prior to the coiled tubing being RIH. Optionally the cleaning fluid comprises a gel. More optionally the gel is flowed, e.g. pumped into the well, at depth, when the second pressure is applied, to hold the debris in suspension. The method may include the step of RIH to move the suspended debris into the coiled tubing. Optionally the method includes the step of retaining the suspended debris in the coiled tubing by use of the fluid control valve. The check valves may then be moved to the closed configuration typically by applying the third pressure. Optionally the coiled tubing is POOH and the suspended debris is typically then released from the space between the check valves and the flow control valve.

According to a second aspect of the present invention there is provided a bottom hole assembly for running on coiled tubing, the assembly comprising a selective check valve including a sleeve which can be positioned through the check valve and hold it in an open configuration; and a flow control valve, the flow control valve comprising: a substantially cylindrical body having an internal bore providing an inlet at a first end and an outlet at an opposing end; a flapper valve located in the internal bore towards the outlet, the flapper valve being arranged to open when a predetermined pressure is applied from the outlet towards the inlet and to allow fluid flow through the flapper valve from the outlet to the inlet; a hollow bore piston arranged adjacent the flapper valve in the internal bore, the piston including at least one port in fluid communication with the hollow bore; and wherein when a pressure is applied to the piston, the piston moves towards the flapper valve and the ports align with at least one fluid flow path arranged in the body, bypassing the flapper valve, and exiting in the internal bore between the flapper valve and the second end.

Typically the piston is biased, optionally in a direction away from the flapper valve towards the inlet, and typically by means of a resilient device such as a spring. The port on the piston can be a single port or can be more than one port, which can optionally be arranged perpendicularly to the hollow bore. Some other orientations of piston ports are possible within other embodiments of the invention. Optionally, the pressure is applied to the piston from the first end.

In one embodiment, the piston has two sealed areas of different sizes and motive force is applied to the piston as a result of the pressure acting on the two sealed areas, thereby generating differential forces on each, and thereby typically resulting in a net axial force applied to the piston to move it within the bore.

In one embodiment, the flow control valve provides a two-way valve which is a check valve in a first direction and a bypass to the check valve which can be opened by a predetermined fluid pressure in the opposite direction. Optionally the pressure is determined by the force required to compress a resilient member such as a spring member between the piston and the body to bias the piston away from the flapper valve. Different strengths of spring can be provided to permit adjustment of the pressure required to overcome the force of the spring.

Optionally flow control valve is located below the selective check valve and the bottom hole assembly is located towards an end of a coiled tubing to be used in the method according to the first aspect.

Optionally the bottom hole assembly also comprises one or more of a group comprising: a drop ball disconnect, a kelly valve and a mule shoe guide.

Certain embodiments of the invention permit debris removal in a well-bore by reverse circulation through coiled tubing, typically in an underbalanced or low formation pressure well.

Embodiments of the present invention will now be described, for example only, with reference to the following drawings of which:

FIGS. 1( a) and (b) are schematic illustrations of a bottom hole assembly on (a) coiled tubing RIH and (b) in part form, in accordance with a method according to an embodiment of the present invention;

FIGS. 2( a) and (b) illustrate cross-sectional views through a selective check valve in a (a) closed configuration and (b) locked open configuration as applied in a method according to an embodiment of the present invention; and

FIGS. 3( a)-(c) illustrate cross-sectional views through a flow control valve in a (a) closed configuration, (b) open with fluid flow from an inlet to an outlet and (c) open with fluid flow from an outlet to an inlet, according to an embodiment of the present invention.

Reference is initially made to FIG. 1( a) of the drawings which illustrates a bottom hole assembly, generally indicated by reference numeral 10, run in a well bore 12 on coiled tubing 14, for the removal of debris 16, according to an embodiment of the present invention.

In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawings are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown in the interest of clarity and conciseness. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce the desired results.

The following definitions will be followed in the specification. As used herein, the term “wellbore” refers to a wellbore or borehole being provided or drilled in a manner known to those skilled in the art. The wellbore may be ‘open hole’ or ‘cased’, being lined with a tubular string. Reference to up or down will be made for purposes of description with the terms “above”, “up”, “upward”, “upper”, or “upstream” meaning away from the bottom of the wellbore along the longitudinal axis of a work string and “below”, “down”, “downward”, “lower”, or “downstream” meaning toward the bottom of the wellbore along the longitudinal axis of the work string. Similarly ‘work string’ refers to any tubular arrangement for conveying fluids and/or tools from a surface into a wellbore. In the present invention, coiled tubing is the typical work string.

FIG. 1( a) illustrates the bottom hole assembly 10 connected to a string of coiled tubing 14 extending to the surface 18. The coiled tubing 14 has a bore 20 for the passage of fluids. The bottom hole assembly 10 and string 14 form an annulus 26 with the wellbore 12. A surface pump 22 at the surface pumps the fluids downhole. A valve 24 associated with the surface pump 22 provides a first path directing fluids down the bore 20 and a second path directing fluids down the annulus 26.

The present embodiment of a bottom hole assembly 10 generally comprises a check valve 42, an optional disconnect sub 44, an optional kelly valve 46, and a flow control valve 50, typically connected to a muleshoe 52 at the lower end of the assembly. Referring now to FIG. 1( b), the assembly 10 is shown in greater detail. The assembly 10 has a substantially cylindrical body having an upper end 32 adapted for connection to the coiled tubing 14. The upper end 32 includes an inlet 34 located centrally which opens into a bore 36 running through the body to an outlet 38 at a lower end 40 of the assembly 10.

The inlet 34 forms a part of a selective check valve 42 which will be described in greater detail herein after with reference to FIGS. 2. Connected to a lower end of the check valve 42 is an optional drop ball disconnect sub 44, and below that is an optional kelly valve 46. The drop ball disconnect 44 is as known in the art and allows the bottom hole assembly to be separated if the lower elements become stuck in the wellbore 12. When disconnected, a fishing neck 48 is left exposed in order to retrieve the lower elements later.

The kelly valve 46 allows the assembly 10 to be separated when recovered at the surface to remove the lower parts before fluid is recovered from the upper parts.

Connected below the kelly valve 46 is a flow control valve 50, which will be described in greater detail herein after with reference to FIG. 3. The bottom hole assembly 10 and string typically terminate with a muleshoe guide 52 as is known in the art for guiding a coiled tubing into a wellbore. The outlet 38 is arranged at the lower end 40 of the muleshoe guide 52.

The embodiment of the bottom hole assembly 10 gives the operator the ability to safely remove debris by reverse circulating from under-balanced and wells which have a low formation pressure. The embodiment of the assembly 10 can be used to continuously reverse circulate debris into the coil or if required can be used as an extended bailer tool with the coil itself acting as the debris chamber.

Reference is now made to FIGS. 2( a) and (b) of the drawings which illustrate a selective check valve 42 incorporating two flapper check valves 52, 54. In this embodiment of a selective check valve 42 there is arranged a hollow bore piston 56 slidable within the bore 36. An annular surface 58 at the upper end of the piston is exposed to fluid pressure entering the inlet 34. The piston 56 is biased towards the inlet 34 by a resilient device in the form of a spring 60 located in an annular chamber 62, bounded by a first shoulder 64 located on the body of the check valve 42 and a second shoulder 66 located on the piston 56 that is movable within the body of the valve 42.

At its upper end adjacent to the annular surface 58, the piston has an upper sliding block 56 b, which bears on its outer surface at least one (in this case two) o-ring seals 56 s which seal the annular space between the body of the valve 42 and the upper sliding block 56 b. The sealed surface area of the piston at the upper sliding block is substantially the full bore of the valve 42.

Below the upper sliding block 56 b, the inner surface of the body of the valve 46 bears at least one lower o-ring seal 59 s, which is sealed to the outer surface of the piston 56 in a similar manner to the upper sliding block 56 b. However, the sealed surface area of the piston at the lower seal 59 s is substantially less than the sealed surface area of the upper sliding block 56 b.

The upper sliding block slides with the piston 56, and the seals 56 s and 59 s are adapted to maintain their seal between the piston and the body of the valve 42 throughout the sliding movement of the piston 56. The area axially between the seals 56 s, 59 s is vented through a port extending through the sidewall of the body of the valve 42.

Mounted upon the outer surface of the piston 56 is a cam arrangement comprising a sleeve 68 with a continuous or semi-continuous slot machined upon its outer surface. As is known in the art, such a slot may be referred to as a J slot or groove, through which one or more pins 70, 72 affixed to an inner surface of the body 30, control the axial and rotational movement of the piston 56. At a lower end 76 of the piston 56, there are arranged a plurality of apertures 74 through the wall of the piston 56 forming a sleeve 80 at the lower end 76. Such apertures 74 allow fluid to flow radially outward from the bore of the piston, through the sleeve 80. The diameter of the sleeve 80 is selected to fit through the check valves 52, 54 when they are in an open configuration.

When the valve is to be operated to move the sleeve 80 though the check valves 52, 54 and keep them open, the pressure within the bore 36 is increased by pumping fluid into the bore 36 from the surface. Below the valve 42, the bore 36 is closed by a flapper valve in the flow control valve 50 (described below) so the pressure within the bore rises as a result of the topside pumping. As the pressure within the bore increased, the piston 56 experiences the same pressure at each of its upper and lower seals 56 s, 59 s, but because each sealed area has a different sealed piston area, the force applied at each seal 56 s, 59 s is different. The upper sliding block 56 b has the larger piston area, and so there is a net downward force on the piston 56 as a result of the increased pressure within the bore 36.

As can be seen with the aid of FIG. 2( b), pressure increases move the piston 56 downwards against the force of the spring 60, which compresses as shown in FIG. 2 b, with the pins 70, 72 travelling along elongate slots on the sleeve of the cam arrangement 68. The sleeve 80 moves down through the flapper valves 52, 54 thereby opening them and allowing fluid to flow up through the bore 36 of the assembly 10 without closing the valves 52, 54. Thus reverse circulation can be achieved, as the flapper valves 52, 54 are held open against the pressure of the fluid flowing up the bore 36. When the pressure is bled off, the pins 70, 72 will typically divert and locate into shorter slots or ‘J’ profiles in the cam arrangement 68, by virtue of the biasing action of the spring 60, thereby locking the sleeve 80 axially in the FIG. 2 b position, straddling the flapper valves 52, 54 and keeping them open. Any fluid pressure from fluid being reverse circulated up the bore 36 from the outlet 38 towards the inlet 34 will pass the valves without closing them. During this the sleeve 80 is kept locked in position straddling the valves 52,54 and keeping them open, by virtue of the force of the spring 60 which keep the pins 70, 72 located in the shorter slots.

Note that pressure can be bled off after the pins have locked in the shorter slots.

To release the sleeve 80, pressure must be applied again to the piston to move the piston down by the differential piston effect described above relative to the stationary pins 70, 72, so that the pins 70, 72 travel along the slots under the force of the pressure against the bias of the spring, thereby moving clear of the short slots or ‘J’ and into the long slots that allow longer axial upward travel of the piston 56. The piston 56 moves axially upward under the force of the spring 60, so that the sleeve 80 moves clear of the valves 52, 54. The pins 70, 72 travel along the long elongate slots with the aid of the spring 60 bias to guide the axial movement of the sleeve 80 clear of the flapper valves 52, 54 and allow the valves 52, 54 to return to their closed position shown in FIG. 2 a. Typically the pressure required to move the piston 56 against the spring force is selected to be lower than the pressure required to close the check valves 52, 54. In this way if a sudden differential pressure is created between the bore 20 of the coiled tubing 14 and the bottom hole assembly 10, such a kick back during RIH or POOH from the formation of the wellbore 12, will automatically shut the check valves 52,54 to provide a protective barrier.

The selective dual flapper check valves 42 thus allow the operator to ‘lock open’ the check valves 52, 54 using a relatively low pressure (approx. 500 psi). Locking the check valves 52, 54 enables the debris 16 to be reverse circulated into the coiled tubing 14 while at depth. Then using pressure again the check valves 52, 54 can be cycled back to the normal running position prior to POOH.

A further flapper valve 90 is found in the flow control valve 50, as illustrated in FIG. 3. This flapper valve 90 is configured in an opposite direction to the check valves 52, 54 in the selective check valve 42. In this way, the flow control valve allows fluid to flow unimpeded through the valve 90, when there is sufficient fluid pressure from the outlet 38 towards the coiled tubing 14. Those in the art will recognise that this will occur during reverse circulation when fluid is pumped down the annulus 26 and returns up the bore 36. Like the selective check valve 42, the flow control valve 90 also has a hollow bore piston 92 within the bore 36.

Like the piston 56, the annulus between the piston 92 and the body of the valve 50 is sealed by upper and lower seals 93 u, and 93 l. The sealed surface area of the piston 92 at the upper seal 93 u is greater than the sealed surface area at the lower seal 93 l. Thus the piston 93 incorporates a differential piston area similar to the piston 56, and application of pressure to the piston 92 causes it to move axially within the bore 36 as a result of the differential forces applied at each of the sealed areas 93 u, 93 l.

The piston 92 is biased towards the inlet 96 by a resilient device in the form of a spring 98. The spring 98 is held in compression between a first shoulder 102 located on the body of the valve 50 and a second shoulder 104 located on the sliding piston 92. A chamber 100 contains the spring 98 and is open to the inlet 96 when pressure is applied to the piston 92. This pressure can act through the chamber 100 on a sleeve 112 located around the piston 92, below the end surface 102. The sleeve 112 has a differential piston area between lower and upper seals also. Towards a lower end 106 of the piston 92, there are arranged a plurality of radial ports 108 through the wall of the piston 92. These ports 108 are initially sealed by the sleeve 112 locating over them.

As can be seen in FIG. 3( b), pressure is increased in the bore 36 and is applied to the piston 92. This pressure is typically greater than the pressure required to move the piston 56 of the selective check valve 42. When thus pressurised, the piston 92 moves downwards by virtue of the differential piston area explained above. Passage of the piston 92 is stopped when a ledge 114 on the piston 92 abuts a ledge 116 on the body 30. At the same time pressure acts upon the sleeve 112 moving it downwards with the piston 92 over a shortened distance to stops 118,120 arranged on the sleeve 112 and body respectively. The ports 108 thus become exposed from the sleeve 112 and align with side ports 122 machined through a portion of the body to access a substantially cylindrical channel 124 arranged within the body to bypass the flapper valve 90. Ports 126 connect the channel 124 with the bore 36 below the end 128 of the flapper valve 90.

Thus the flow control valve 50 provides a two way valve which opens at a pre-determined pressure (adjustable e.g. from 1000-4500 psi) to allow flow from the BHA 10 into the wellbore 12 or free flow from the wellbore 12 into the bore 20 of the coiled tubing 14 when the check valves 52, 54 are locked open. The purpose of the flow control valve is two fold; when closed, it allows the check valves 52,54 to be opened or closed without pumping any fluid into the wellbore 12 and when the system 10 is used as a bailer, to be described later, it ensures that the debris 16 is retained within the coiled tubing 14.

A method of operating the controlled debris removal system will now be described with reference to the aforementioned Figures and according to an embodiment of the present invention.

At surface 18 the bottom hole assembly 10 is assembled and connected to an end of the coiled tubing 14. The system 10 can then be checked before being RIH. Whilst running in the hole the check valves 52, 54 are typically held in the conventional, closed position. This can be checked at surface prior to RIH or at any time in the well by applying minimal pressure to the coiled tubing annulus 26 to ensure that there is no pressure increase within the assembly 10 above the valves 52, 54. No pumping is possible unless the opening pressure of the flapper valve 90 within the flow control valve 50 is exceeded. The flow control valve 90 can be tested in this manner. A bleed off port 130 can be accessed between the selective check valve 42 and the drop ball disconnect 46 to relieve any trapped pressure between the check valves 52, 54 and the inverted check valve 90.

With all the check valves 52, 54, 90 set in the closed configuration, the coiled tubing 14 and BHA 10 is RIH until the required depth is reached for removal of debris 16. On RIH the selective check valve 42 will be in the closed position illustrated in FIG. 2( a) and the flow control valve 50 will be in the closed position illustrated in FIG. 3( a).

Once at depth the check valves 52, 54 can be locked open by applying the required 500 psi pressure to the BHA 10 to drive the piston 56 through the valves 52, 54 and then bleeding off pressure to lock the pins 70, 72 within a shortened J slot in the cam arrangement 68, as described with reference to FIG. 2( b). This will result in the check valves 52, 54 being locked open and reverse circulating can begin. Here fluid is pumped down the annulus 26 and returns up the bore 36 to the surface 18. The debris 16 is caught in the fluid flow and is carried up the bore 36 for disposal.

Removing debris using reverse circulation is advantageous for many reasons. In particular, because the coiled tubing flow bore 36 has a smaller cross-sectional flow area than the annulus 26 cross-section, the flow rates required to keep the debris suspended in the fluid can be proportionately reduced to achieve the same velocity. The lower flow rate is desirable to maintain the well in an underbalanced condition i.e. the small pump rate down the annulus creates a pressure at the BHA which is lower than the formation pressure to prevent the pumped fluid being forced into the formation. The lower flow rate is also desirable to reduce erosion within the coiled tubing, and reduce the likelihood that the coiled tubing will be damaged or will collapse due to differential pressure. Further, the circular cross section of the coiled tubing flow bore 36 provides a more efficient flow path than the annular cross-section of the wellbore annulus 26, and minimizes “dead spaces”, i.e. areas of blockage where little or no flow can get through, which is where the debris may become trapped. Additionally, the flow area in the coiled tubing flow bore is typically the same size along the entire flow path, whereas the wellbore annulus 26 increases in size from the bottom to the top of the wellbore, thereby increasing the likelihood that cuttings will fall out of suspension in the larger areas, or on ledges.

The check valves 52, 54 will remain open as long as required for satisfactory wellbore 12 cleaning to be achieved. It is noted that during reverse circulation the flapper valve 90 also opens, as illustrated in FIG. 3( c), allowing the debris-laden fluid to enter the bores 36,38 unimpeded.

When the coiled tubing 14 and BHA 10 are required to be POOH, the check valves 52, 54 are typically cycled back to their conventional closed position. With the pumping switched at surface 10 to travel down the bore 36, the flapper valve 90 of the flow control valve 50 will automatically close. Increasing pressure to approx. 500 psi within the bore 36 will slide and turn the piston 56 in the cam arrangement 86 and return the pins 70,72 to the elongate slot, so that the force of the spring moves the piston 56 back up toward the inlet 34 and the sleeve 80 is withdrawn, clear of the flapper valves 52,54. The valves 52, 54 will close and the selective check valve will return to the position illustrated in FIG. 2( a). This can be confirmed again by pressurising up the coiled tubing annulus 26. The BHA 10 can now be safely pulled from the wellbore 12. During POOH, each piston 56, 92 is prevented from moving downwards by the fluid now trapped between the check valve 54 and the flapper valve 90. This fluid can be released at surface through the bleed port 130.

A further embodiment of the method of the present invention provides collection of the debris 16 by using the BHA 10 as a bailer. In this embodiment, the system 10 is used in a very similar way to the above with two distinct changes. The debris 16 is not continuously reversed into the coiled tubing bore but a predetermined volume is pumped into the bore 36 and that volume is pulled out of the hole with the BHA 10 and held in the bore 36 by the flow control valve 90. The second difference is that prior to RIH, gel is introduced into the coil tubing 14 prior to attaching the BHA 10.

Whilst RIH, as illustrated in FIG. 3( a), the gel is retained within the bore 36 by the flow control valve 50, and more particularly by the flapper valve 90. Once at depth, pressure is increased through the coiled tubing 14 to open the flow control valve 50 as illustrated in FIG. 2( b). This allows the gel to be pumped through the bypass channel 124 into the wellbore 12 where it entrains the debris 16, holding it in suspension. At the same time the selective check valves will cycle open due to the increased pressure. Once the desired volume of gel has been pumped into the wellbore 12 and has entrained the debris, the pressure can be bled off and the check valves will be locked open as shown in FIG. 2( b).

Further RIH will allow the gel, including the suspended debris, to enter the BHA 10 and coiled tubing 14, through the inlet 38 and through the now freely opened flapper valve 90 of the flow control valve 50. This is as illustrated in FIG. 3( c). Once inside the BHA 10, the debris-laden gel will be retained inside by the check valve nature of the flapper valve 90. The pressure can then be increased to 500 psi to cycle the selective check valve 42 to the closed position prior to POOH, as illustrated in FIGS. 2( a) and 3(a).

At surface 10, the debris-laden gel can be forced out of the BHA 10 by inserting fluid or gas into the outlet 38 and allowing the gel to be released through the bleed off port 130. It is noted that the gel trapped between the check valve 54 and the flapper valve 90, will be at the pressure required to move the sleeve 80. This will typically be around 500 psi and thus the gel will be released from the BHA 10, at surface 18, when the port 130 is opened.

It will be appreciated by those skilled in the art that various modifications may be made to the invention herein described without departing from the scope thereof. For example, motors or other tools can be incorporated into the bottom hole assembly. Further embodiments of check valves which can be locked open may be used. The tools may also be located in any order provided that the flow control valve is located below the check valve. A clear passage for a drop ball is desirable but not essential in the event that the disconnect is required.

Embodiments of the present invention allow the use of coiled tubing in reverse circulation operations. One significant advantage of this is that the normally small diameter of coiled tubing does not limit the efficiency of the process, as high volumes can be pumped down the annulus, typically at low flow rates, causing minimum disruption to well infrastructure and equipment, and the high volumes can wash significant quantities of debris into the low volume small diameter coiled tubing at the bottom of the well, which permits high flow rates in the debris-laden fluid being recovered through the narrow bore coiled tubing. 

1. A method of removing debris from a well, the method comprising: running into the hole (RIH) to required depth with coiled tubing having at least one check valve arranged in a closed configuration; at depth, applying a first pressure to move a sleeve through the at least one check valve to hold the check valve in an open configuration; applying a second pressure to open a flow control valve located on the coiled tubing below the check valve(s); reverse circulating a fluid to entrain debris in the wellbore and bring it to surface through the coiled tubing; releasing the sleeve from the at least one check valve; and pulling out of the hole (POOH) with the at least one check valve returned to the closed configuration.
 2. A method as claimed in claim 1 wherein the second pressure to open the flow control valve is greater than the first pressure.
 3. A method as claimed in claim 1, wherein the first pressure is bled off to lock the at least one check valves in the open configuration.
 4. A method as claimed in claim 1, including the step of closing the flow control valve at depth to retain the debris within the coiled tubing prior to pulling out of hole.
 5. A method as claimed in claim 1, including the step of receiving a predetermined volume of debris-laden fluid into the coiled tubing.
 6. A method as claimed in claim 1, including the step of introducing a cleaning liquid to the coiled tubing between the flow control valve and the check valves prior to the coiled tubing being run in hole.
 7. A method as claimed in claim 6, wherein the cleaning liquid comprises a gel.
 8. A method as claimed in claim 7, including the step of flowing the gel into the well, at depth, when the second pressure is applied, to entrain the debris and hold it in suspension.
 9. A method as claimed in claim 8, including the step of running the coiled tubing in hole (RIH) to cause the suspended debris to flow into the coiled tubing.
 10. A method as claimed in claim 9, including the step of retaining the suspended debris in the coiled tubing by use of the fluid control valve.
 11. A method as claimed in claim 10, including the step of moving the check valves to the closed configuration by applying pressure.
 12. A method as claimed in claim 8, including the step of pulling the coiled tubing out of hole (POOH) and releasing the suspended debris from the space between the check valves and the flow control valve.
 13. A bottom hole assembly for running on coiled tubing, the assembly comprising: a selective check valve including a sleeve which can be positioned through the check valve and hold it in an open configuration; and a flow control valve, the flow control valve comprising: a substantially cylindrical body having an internal bore providing an inlet at a first end and an outlet at an opposing end; a flapper valve located in the internal bore towards the outlet, the flapper valve being arranged to open when a predetermined pressure is applied and to allow fluid flow through the flapper valve from the outlet to the inlet; a hollow bore piston arranged adjacent the flapper valve in the internal bore, the piston including at least one port in fluid communication with the hollow bore; and wherein when a pressure is applied to the piston, the piston moves towards the flapper valve and the at least one port aligns with at least one fluid flow path arranged in the body, bypassing the flapper valve, and exiting in the internal bore between the flapper valve and the second end.
 14. A bottom hole assembly as claimed in claim 13, wherein the piston is biased in a direction away from the flapper valve towards the inlet, by means of a resilient device.
 15. A bottom hole assembly as claimed in claim 13, wherein the pressure is determined by the force required to compress the resilient device between the piston and the body.
 16. A bottom hole assembly as claimed in claim 13, wherein the flow control valve is located below the at least one selective check valve and the bottom hole assembly is located towards an end of a coiled tubing.
 17. A bottom hole assembly as claimed in claim 13, comprising one or more of a group comprising: a disconnect sub, a kelly valve and a mule shoe guide.
 18. A bottom hole assembly as claimed in claim 13, having a plurality of check valves.
 19. A bottom hole assembly as claimed in claim 13, wherein the at least one check valve comprises a dual flapper arrangement.
 20. A bottom hole assembly as claimed in claim 13, wherein the flow control valve is a two-way valve opening at the predetermined second pressure to allow flow of fluid from the coiled tubing to the wellbore or to allow free flow from the wellbore to the coiled tubing when the at least one check valve is locked open.
 21. A bottom hole assembly as claimed in claim 13, wherein the flow control valve incorporates a separate reverse check valve.
 22. A bottom hole assembly as claimed in claim 13, wherein at least one piston is moved by pressure applied to two sealed areas of the piston, the sealed areas having different surface areas. 